The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the measurement and imaging of particle motion.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It is well known that NMR can be used to detect and image the movement of spins. As disclosed in U.S. Pat. No. Re. 32,701 entitled, xe2x80x9cNMR Scanner with Motion Zeugmatographyxe2x80x9d, acquired NMR signals can be sensitized to detect moving spins by applying a bipolar magnetic field gradient at the proper moment in each NMR measurement sequence. The phase of the resulting NMR signal measures the velocity of spins along the direction of the motion sensitizing magnetic field gradient. With more complex motion sensitizing magnetic field gradients, higher orders of motion, such as acceleration and jerk, can also be measured with this method.
It has been found that MRI imaging can be enhanced when an oscillating stress is applied to the object being imaged in a method called MR elastography. The method requires that the oscillating stress produce shear waves that propagate through the organ, or tissues to be imaged. These shear waves alter the phase of the NMR signals, and from this the mechanical properties of the subject can be determined. In many applications, the production of shear waves in the tissues is merely a matter of physically vibrating the surface of the subject with an electromechanical device such as that disclosed in U.S. Pat. No. 5,592,085. For example, shear waves may be produced in the breast and prostate by direct contact with the oscillatory device. Also, with organs like the liver, the oscillatory force can be directly applied by means of an applicator that is inserted into the organ.
A number of methods have been devised to produce shear waves inside a subject without direct physical contact. For example, ultrasonic energy can be directed into the subject and modulated to produce forces that generate shear waves inside the subject as described in U.S. patent application Ser. No. 08/758,879 filed Dec. 2, 1996 and entitled xe2x80x9cAcoustic Force Generator For Detection, Imaging And Information Transmission Using The Beam Signal Of Multiple Intersecting Sonic Beamsxe2x80x9d. Also, in some applications, the entire subject can be shaken (e.g. the human head) and inertial forces are generated inside the subject that produce shear waves as described in co-pending U.S. patent application Ser. No. 09/057,405 filed on Apr. 8, 1998 and entitled xe2x80x9cInertial Drive Device For MR Elastographyxe2x80x9d.
The present invention is a method for imaging a subject with an MR imaging system in which an oscillating voltage produces an electric field in the subject that moves charged particles, or produces a current through the subject that moves ions. An alternating magnetic field gradient produced by the MRI system during an image acquisition oscillates in synchronism with the applied voltage to sensitize for synchronized spin motion. The moving charged particles or ions and surrounding spins in the subject which may be set in synchronous oscillating motion, produces a phase shift in the acquired NMR signals which may be seen in reconstructed phase images.
A general object of the invention is to image the motion of charged particles or ions in a subject. By moving the particles with an oscillating electric field or current that is synchronized with an oscillating imaging gradient, the resulting movement of gyromagnetic spins may be imaged. The charged particles or ions may themselves be gyromagnetic such that their motion is seen directly in the reconstructed phase image. In addition, movement of the charged particles or ions may impart motion to surrounding gyromagnetic spins, and the observed motion of these surrounding spins in the reconstructed image provide an indirect indication of charged particle or ionic motion.
Another object of the invention is to produce shear waves in the subject material from which its mechanical properties can be measured and imaged. The charged particles or ions may be dispersed throughout the subject material and set in motion by the applied oscillating voltage. The motion of the particles applies a stress to surrounding material which produces shear waves that can be imaged with the MRI system. The location and strength of the shear waves can be tailored by the number and distribution of the particles throughout the subject material and by the strength and location of the applied voltage.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.